The invention relates to a glass-plastic composite ophthalmic lens.
Composite lenses which contain a thin layer of glass bonded by a transparent adhesive to a relatively thick lens made of transparent plastic material have already been proposed in the patent literature.
For example, EP-A 0 182 503 [which corresponds to U.S. Pat. No. 4,679,918 (Ace)] describes a composite lens containing a glass lens, for example, a photochromic glass lens, and a lens made of plastic material whose radii of curvature of the surfaces facing each other are different; these lenses are joined by a relatively thick intermediate layer of adhesive elastomer material of optical quality capable of great elongation, for example, approximately 400%. The silicone elastomers are the only adhesive elastomer materials described.
The problem posed by the manufacturing of composite lenses of the above-mentioned type and which the aforementioned documents attempt to solve is that of obtaining such composite lenses which are capable of resisting the variations of temperature to which these lenses are subjected when in service. This problem is difficult to overcome because of the great difference existing between the thermal expansion coefficients of the glass and of the plastic material, which makes the differential expansion high between the plastic material and the glass and causes the thin layer of glass to break when the lens is subjected to temperature variations when one uses an ordinary adhesive, such as an epoxy resin of optical quality, to bind the glass layer to the plastic lens.
The problem considered above is particularly hard to solve when the plastic lens consists of poly(diethylene glycol bis(allyl carbonate)), which is a plastic material widely used in the manufacturing of ophthalmic lenses, sold by the company PPG Industries, Pittsburgh, Pa., under the commercial designation CR39.RTM.. CR39.RTM. actually exhibits a linear thermal expansion coefficient which is more than 10 times higher than that of mineral glasses of optical quality.
The solution proposed by the aforementioned documents in an attempt to solve this problem is based on the use, as adhesive, of a silicone elastomer with a high elongation at rupture in order to minimize the stresses generated in the composite lens by the differential thermal expansion between the glass and the plastic material.
This solution is nevertheless not entirely satisfactory because it does, not take into account the long-term effect of humidity which particularly tends to degrade the properties of transparency and elongation of the adhesive. In effect, the adhesives of the silicone type have a pronounced hydrophobic nature and do not dissolve in water. Any water present forms a two-phase system with the adhesive, which gives the layer a milky and diffusing, and therefore redhibitory, aspect. Moreover, the theoretical approach on which the proposed solution is based, is inexact. In effect, the aforementioned documents consider that the phenomenon of thermal expansion to which the lens is subjected is that of the simple variation of the diameters of two flat disks, free to expand, whereas one is in the presence of elements with curved surfaces. The aforementioned documents also neglect the effect that can occur when one mounts composite lenses on a frame, for example a metallic frame, of glasses. Mounting the lenses in effect is usually done on the plastic part of the lens which limits the increase of the diameter of the lenses when they are exposed to a relatively high temperature. Consequently, the radius of curvature of the part made of plastic material has a tendency to decrease (bulging effect), with the result that the stresses on the edges of the lenses increase more than could be expected from a simple calculation of variation of the diameters. The humidity also has a similar effect to that of thermal expansion, which is therefore added to that of thermal expansion, aggravating the destructive phenomenon observed.
In any case, the applicants have no knowledge that the composite lenses described in the aforementioned documents have been commercialized, and this is probably because of the fact that these lenses do not have an entirely satisfactory behavior in the long term.
The applicants, therefore, undertook intensive research and found that an essential property for the material of the intermediate adhesive layer, besides a great elongation capability, was a low elastic modulus, namely, an elastic modulus of at most 1.0 MPa with 100% elongation. Therefore, the applicants, research was directed to minimizing the stresses undergone by the thermosetting polyurethanes of optical quality, which normally have an elastic modulus much higher than the maximum value mentioned above, by modifying their composition so as to lower their elastic modulus below this value.